This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Swedish Application No. 0101836-5 filed on May 23, 2001, the entire content of which is incorporated herein by reference.
This invention generally relates to continuous casting machine. More particularly, the invention pertains to a method for detecting an at least partly bulging portion of an elongated material produced in a continuous casting machine.
A continuous casting machine produces steel material from molten steel. This steel material can, for example, be used as a starting material in rolling processes for producing sheet metal used in, for example, vehicles.
In continuous casting machines, molten steel flows from a ladle and down in a tundish from which it is transported further down into a mold. In the mold, which is water-cooled, the slab of continuous cast material begins to form a solid shell. Then, the slab is continuously transported in between two curved tracks (a first track and a second track) by a large number of rollers arranged in segments which continue to shape and cool the slab to the final thickness of the steel material. At the end of the tracks, the material is cut into suitable pieces. The cooling can be achieved by spraying water onto the slab and the rollers.
The rollers of the continuous casting machine are mounted with their axes substantially perpendicular to the longitudinal extension of the curved tracks. To lead and support the slab of continuous cast material, the rollers are arranged in pairs, each comprising a roller from the first track and a roller from the second track.
Further, the rollers are rotatably mounted in supporting members at each end of the rollers and due to the length of the rollers, and the load on the rollers, the rollers are generally split into at least two roller portions. These roller portions are either independently mounted in supporting members or non-rotatably provided on a common shaft, with the shaft being rotatably mounted in supporting members. The supporting members can for instance be rolling bearings or sliding bearings with corresponding bearing housings.
Considering the solidification process of the material being cast, the process starts at the slab surface and a thin layer of substantially solidified material is formed around the liquid core. Further cooling results in the side edges of the slab slowly being solidified, while the center of the slab is still substantially liquid except for the surface layer. Yet further, the core of liquid material will slowly decrease and finally the core is entirely solidified. During the solidification, when the material is cooled down, the material will generally shrink as hot metal has a larger volume than cold metal.
This shrinkage can result in a problem in that one of the conditions that must be fulfilled to obtain a cast material of high quality and relatively even thickness is that the rollers of the first track and the rollers of the second track must be able to correctly support the slab and control the thickness of the slab throughout the entire process. Thus, the mutual distance between the two tracks must correspond to the desired thickness of the slab at every point during the process. Such a set up of the machine is generally quite difficult to obtain.
However, it can also be established that the two tracks converge towards each other. This means that in the upper portion of the machine where the slab is very hot, the mutual distance between the rollers of the first and second track is larger than at a location further down in the machine where the slab has been cooled off, as the slab in that position has shrunk somewhat.
If the mutual distance between the tracks is not correct, i.e. if the tracks are not converging towards each other in an accurate way, the thickness of the material being cast will not be uniform. Considering a pair of rollers where the mutual distance between the rollers is too large, the cross-sectional profile of the material being cast will at least partly bulge outwards, i.e. will have a convex profile where the middle portion of the slab will be thicker than the side edges. This is due to the fact that the sides have started to solidify, while the centre of the slab is still liquid. If there is no pressure from the rollers, the inner pressure of the material flowing down from the mold will force more material into the liquid centre of the slab and the middle portion of the slab will therefore expand. The deformation can lead to depressions near the slab corners, which can lead to longitudinal corner cracks.
On the other hand, when the mutual distance between the rollers is too small, the material will at least partly bulge inwards as it is squeezed and rolled between the rollers. The roll motion forces some of the material in the molten core to flow back against the transportation direction. Hence, there will be too little material left in the middle of the slab when it is cooled off and the slab profile will be concave. Furthermore, this roll motion exerts dynamic forces to the rollers and the supporting members, which together with the load of the slab and the weight of the roller, can lead to extremely high loads in the supporting members, which in turn can lead to failures.
From the discussion above, it can be seen that it would be desirable to be able to detect an incorrect mutual distance between the tracks of rollers so that a correct adjustment of the rollers can be made.
Hereinafter, an incorrect mutual distance between the first track and the second track is denoted as an erroneous convergence between the tracks. A method according to one aspect of the present invention involves a method for detecting an at least partly bulging portion of an elongated material produced in a continuous casting machine and involves measuring the radial load exerted by the material on each supporting member of the roller portions of a roller, comparing the radial load values of the supporting members arranged in the ends of two adjacent roller portions facing away from each other with those of the supporting members arranged in the ends of the two adjacent roller portions facing each other, and establishing the presence of an at least partly bulging portion of the elongated material where the divergence between the load values of the supporting members arranged in the ends of the two adjacent roller portions facing away from each other and the supporting members arranged in the ends of the two adjacent roller portions facing each other is exceeding a predetermined value.
If the load values of the supporting members arranged in the ends of the two adjacent roller portions facing away from each other are higher than the load values of the supporting members arranged in the ends of the two adjacent roller portions facing each other, it can be established that the mutual distance between the tracks is too small. If instead the load values of the supporting members arranged in the ends of the two adjacent roller portions facing away from each other are lower than the load values of the supporting members arranged in the ends of the two adjacent roller portions facing each other, it can be established that the mutual distance between the tracks is too large.
Thus, if the load values of the supporting members arranged in the ends of the two adjacent roller portions facing away from each other are equal or substantially equal to the load values of the supporting members arranged in the ends of the two adjacent roller portions facing each other, it can be determined that an appropriate mutual distance exists between the tracks.
According to another aspect, a method for detecting an at least partly bulging portion of an elongated material produced in a continuous casting machine comprises transporting the elongated material between two tracks converging towards each other in the continuous casting machine, with each track comprising a plurality of rollers arranged substantially perpendicular to longitudinal extensions of the tracks, and at least some of the rollers being divided into at least two axially adjacent roller portions, with each end of each roller portion being rotatably mounted in a supporting member. The method also comprises measuring a radial load exerted by the material on each supporting member of the roller portions of one at least one of the rollers, comparing the radial loads of the supporting members at the ends of two adjacent roller portions facing away from each other with the radial loads of the supporting members at the ends of the two adjacent roller portions facing each other, and determining that an at least partly bulging portion exists in the elongated material when a divergence between the radial loads of the supporting members arranged in the ends of the two adjacent roller portions facing away from each other and the supporting members arranged in the ends of the two adjacent roller portions facing each other exceeds a predetermined value.